There is great interest concerning the impact of and possible causes of global warming. As is well known, global warming is thought to be caused by the “greenhouse” gas effect where gases such as carbon dioxide, which is emitted by the combustion of fossil fuels, delay the radiation into outer space of the corresponding thermal energy released from the combustion of the fossil fuels. One approach that is being seriously considered at this time to reduce the emission of carbon dioxide is to produce hydrogen as a fuel for fuel cells. Hydrogen powered fuel cells are under development for future applications to electric vehicles and for distributed electrical energy sources. However, the currently known methods of producing hydrogen are very energy intensive, with electrolysis of water being the most energy intensive of the known methods.
The predominant scientific opinion at this time is that global warming is occurring and is caused to a significant extent by human activities. At the same time, demand for fossil fuels by rapidly developing nations with large populations such as China and India is increasing the cost of energy and the potential for even further emissions of greenhouse gases.
Many portions of the world, including the United States, are subject to persistent drought conditions. As a result, there has been an increased interest in improving methods of desalination. The oceans have an average worldwide salinity of 35,000 ppm (3.5%), of which about 30,000 ppm (3.0%) are Na+ and Cl− ions in solution.
Sources of saline water are not limited to the oceans. Underground saline aquifers are located in many portions of the western United States where persistent drought conditions are most severe. Brackish water is generally defined as water having a salt concentration of about 1000 to 8000 ppm as compared to drinking water which is generally considered to range from 250 to 1000 ppm. The theoretical minimum energy requirement to convert seawater to fresh water is given by various sources as ranging from 0.050 to 0.065 KJ/mol (kilojoules per mole).
In general, reverse osmosis is the method most commonly used for desalination of both seawater and brackish water. In reverse osmosis, the salt water is pumped to a high pressure through a tubular membrane such that the salt ions remain trapped in the interior portion of the membrane. Another method of desalination is electrodialysis, wherein a potential difference V across a stack of alternately charge selective membranes causes alternating concentrations of brine and fresh water between the membranes. Anolyte and catholyte are produced at the respective anode and cathode. The anolyte and catholyte are sometimes referred to as electrochemically activated water. Electrochemically activated water, containing either an excess of positive ions or an excess of negative ions, is sometimes used as a biological disinfectant.
A method of desalination which has received increasing interest in the past several years is called capacitive deionization (CDI). Salt water enters the space between two electrodes that are maintained at a potential difference V of about 1.2V so that the electrostatic field forces sodium and chlorine ions into the aerogel, where they are retained, and pure water leaves the space between the electrodes. The ions and other charged particles (such as microorganisms) are attracted to and retained by the electrode of opposite charge. During the application of the potential difference V, the negative electrode accumulates electrons, which are negatively charged, on the surface so that the negative electrode attracts positively charged hydrated ions (cations) such as calcium (Ca), magnesium (Mg) and sodium (Na).
Correspondingly, during the application of the potential difference V, the positive electrode, accumulates positively charged “holes” on the surface so that the positively charged electrode attracts hydrated negative ions (anions) such as chloride (Cl) and nitrate (NO3). Due to the polar structure of water as HOH, the term “hydrated ion” refers to the resulting combination of about six to eight polar water molecules which are attracted by the Coulomb forces to a single ion, be it a positively charged ion or a negatively charged ion, resulting in a cluster ion, as shown in FIG. 1. That is, the H+ polar end of each water molecule is attracted to the Cl− ion while the OH− polar end of each water molecule is attracted to the Na+ ion.
Eventually the electrodes become saturated with the hydrated ions and the electrodes must be regenerated. The applied potential V is removed, and since there is no longer any reason for the ions to remain attached to the electrodes, the ions are released and flushed from the system, producing a more concentrated brine stream. Oftentimes, to speed the regeneration time, the polarity of the applied potential is actually reversed rather than being simply removed. In practice, more than 80% of water fed to a CDI process emerges as fresh, deionized potable water, and the remainder is discharged as a concentrated brine solution containing virtually all of the salts in the feed.
Carbon aerogel may be used as the electrode material for CDI because such a material is stable in harsh chemical conditions and possesses a very high specific surface area (about 100-1000 square meters per gram of aerogel). It is the very high specific surface area of the carbon aerogels which has advanced the state of the art of capacitive deionization. However, carbon aerogel is still costly to produce. Alternative materials such as mesoporous carbon are available or being developed.
It is generally recognized at this time that one of the most challenging aspects of desalination and salinity control is management of the brine concentrate by-product. In both coastal and inland regions, the costs and regulatory requirements associated with concentrate management remains a significant problem.
Therefore, due in part to the high cost of carbon aerogel, and at least partly due to the inherent cost of energy which must be input into existing desalination processes, desalination processes still remain limited in their application. The recent increase in energy costs adversely affects the economics of desalination as well as already well-established sectors of the economy, particularly transportation. The main fuel for transportation, gasoline, has an energy content of about 35 MJ (megajoules) per liter. It is against this energy content against which alternative energy technologies such as solar, wind, biomass (e.g., biodiesel and ethanol), hydrogen for fuel cells, as examples, are sometimes compared against.
It is well known that nuclear energy in the form of nuclear fission and nuclear fusion have energy contents on a per mass basis which greatly exceed that of fossil fuels and also that neither form of energy results in the formation of greenhouse gases. Nuclear energy is released due to the difference in mass of the reactants versus the products. In that the mass of the products is less than the mass of the reactants, the difference in mass is converted to energy according to Einstein's equation, E=mc2, where E is the energy in joules, m is the mass in kilograms, and c is the velocity of light in meters/second (about 3.0×108/meters per second).
Consequently, controlled thermonuclear fusion has been under development for many years. In nuclear fusion, the goal is to overcome the Coulomb forces of repulsion between pairs of like-charged ions, e.g., between pairs of deuterium ions, so that the like-charged ions approach each other closely enough so that the strong force or nuclear force predominates over the Coulomb force of repulsion. At a close enough distance between the like-charged ions, the strong force or nuclear force causes the pair of ions to fuse together to produce a product atom or ion, e.g., helium and other products, that have a total mass which, although heavier than the individual reactant ions, is less than the mass of the pair of reactant ions taken together. The difference in mass of the products is then converted to nuclear energy as described above.
More recently, lasers having intensities as high as 1020 watts per square centimeter have been used to cause a phenomenon known as a Coulomb explosion. The laser first causes an extreme cluster multielectron ionization and then a cluster Coulomb explosion resulting from the forces of repulsion between like charged nuclei. The Coulomb explosion phenomenon is under investigation as a means of achieving deuterium-deuterium nuclear fusion.
However, to date, there is no commercially available means for controlled nuclear fusion. Nuclear fission reactors, while commercially available and feasible, have been hampered by well-known problems involving long construction times, high capital costs and public perception of safety issues. Other alternative energy sources have yet to achieve a degree of commercial application and economic attractiveness sufficient to offset the continued production of greenhouse gases by the combustion of fossil fuels.